The present invention relates to a photoelectric conversion device such as a solar cell or the like, more particularly, to a photoelectric conversion device having a low reflectance with respect to incident light with a wavelength in the vicinity of a wavelength allowing a photoelectric conversion unit to have an optimal spectral sensitivity characteristic so as to improve the conversion efficiency of the photoelectric conversion device. The present invention also relates to a substrate for a photoelectric conversion device.
In a thin film photoelectric conversion device such as a thin film solar cell, a glass sheet with a transparent conductive film (a transparent electrode) may be used in some cases. This thin film photoelectric conversion device is manufactured by forming, on a glass sheet, a transparent conductive film containing tin oxide as a main component, a photovoltaic unit including a photovoltaic layer, and a back electrode of aluminum or the like in this order.
As the transparent conductive film, a fluorine doped tin oxide (hereinafter referred to as xe2x80x9cSnO2:Fxe2x80x9d) film has been used in many cases. This film is more excellent in chemical stability such as plasma resistance or the like compared to a tin doped indium oxide (ITO) film. Therefore, the SnO2:F film does not deteriorate greatly in forming a photovoltaic layer by using a plasma CVD method. A thin film photoelectric conversion device in which an undercoating film is formed between a transparent conductive film and a glass sheet also has been known. This undercoating film functions as a barrier film for preventing an alkaline component from diffusing into the transparent conductive film from the glass sheet. As the barrier film, a silicon oxide film has been used in many cases.
A glass sheet with a transparent conductive film also is used as window glass for buildings. The glass sheet with a transparent conductive film formed thereon suppresses the outflow of heat from an opening of a building as so-called Low-E glass. In this application field, it is important that a natural appearance as window glass is provided. A tin oxide film also is one of the typical transparent conductive films in this field. However, when the tin oxide film is formed to have a thickness effective in suppressing the heat loss from an opening, the problem of interference colors (iridescence) of reflected light comes up. Therefore, JP 3-72586 B discloses the formation of two intermediate layers between a glass sheet and a transparent conductive film. Specifically, it discloses that a tin oxide film with a thickness of about 18 nm and a silicon-silicon oxide mixed film with a thickness of about 28 nm are formed sequentially from a glass sheet side and further a SnO2:F film with a thickness of about 200 nm is formed on those films as a transparent conductive film.
On the other hand, the transparent conductive film used for the thin film photoelectric conversion device is required to have the compatibility between a high light transmittance and a high conductivity. However, these two properties show reciprocal tendencies and thus the compatibility cannot be obtained easily. Thus, a thin film photoelectric conversion device using a transparent conductive film itself as an antireflection film by adjusting the thickness of the transparent conductive film so that a large quantity of light reaches a photovoltaic layer also has been proposed (for instance, xe2x80x9cAmorphous Solar Cellxe2x80x9d by Kiyoshi Takahashi and Makoto Konagai, published by Shokodo).
However, when the transparent conductive film itself is intended to be used as an antireflection film, the thickness of the transparent conductive film is limited, which causes difficulty in controlling the conductivity. Therefore, the improvement in characteristics of the photoelectric conversion device as a whole cannot be expected. Furthermore, in a photoelectric conversion device including a plurality of photovoltaic layers having different spectral sensitivity characteristics, it is difficult to exert an antireflection effect on the plurality of photovoltaic layers merely by adjusting the thickness of the transparent conductive film.
As described in JP 3-72586 B, in the field of window glass for buildings, it also has been proposed to insert a plurality of films between a glass sheet and a transparent conductive film. However, it has not been studied yet to improve the characteristics of a photoelectric conversion device by using an intermediate film between a glass sheet and a transparent conductive film. In order to improve the characteristics of the photoelectric conversion device, consideration also must be given to the spectral sensitivity characteristics of a photovoltaic layer.
The present invention is intended to provide a photoelectric conversion device such as a photovoltaic device in which conversion efficiency is improved by employing a film structure including an intermediate film between a transparent substrate and a transparent conductive film. The present invention also is intended to provide a substrate for a photoelectric conversion device that has the above-mentioned film structure and is effective in improving the conversion efficiency.
In order to achieve the above-mentioned object, a first photoelectric conversion device of the present invention includes a transparent substrate, a transparent conductive film, a photoelectric conversion unit including a photoelectric conversion layer, and a back electrode, which are stacked sequentially from the side on which light is incident. Further, an intermediate film is formed between the transparent substrate and the transparent conductive film. The first photoelectric conversion device satisfies the relationship of R1 less than R2xc3x970.8, wherein R1 represents an average reflectance of the photoelectric conversion device in the wavelength region between (xcexxe2x88x9250) nm and (xcex+50) nm, where xcex (nm) indicates a wavelength of the light allowing the photoelectric conversion layer to have an optimal spectral sensitivity characteristic, and R2 denotes an average reflectance, in the wavelength region, of the photoelectric conversion device that does not include the intermediate film.
A second photoelectric conversion device of the present invention includes a transparent substrate, a transparent conductive film, at least two photoelectric conversion units, and a back electrode, which are stacked sequentially from the side on which light is incident. The at least two photoelectric conversion units include two photoelectric conversion layers in which wavelengths xcex of the light allowing optimal spectral sensitivity characteristics to be obtained are different from each other. Further, an intermediate film is formed between the transparent substrate and the transparent conductive film. The second photoelectric conversion device satisfies the relationships of: R11 less than R12, wherein R11 represents an average reflectance of the photoelectric conversion device in a first wavelength region between (xcexxe2x88x9250) nm and (xcex1+50) nm, where xcex1 (nm) represents one of the wavelengths xcex in one of the two photoelectric conversion layers, and R12 denotes an average reflectance, in the first wavelength region, of the photoelectric conversion device that does not include the intermediate film; and R21 less than R22, wherein R21 represents an average reflectance of the photoelectric conversion device in a second wavelength region between (xcex2xe2x88x9250) nm and (xcex2+50) nm, where xcex2 (nm) represents the other of the wavelengths xcex in the other of the two photoelectric conversion layers, and R22 denotes an average reflectance, in the second wavelength region, of the photoelectric conversion device that does not include the intermediate film.
According to the photoelectric conversion devices of the present invention, since the average reflectances are low in the wavelength regions allowing the photoelectric conversion unit to have a high spectral sensitivity characteristic, the conversion efficiency of the photoelectric conversion devices is improved. Furthermore, since the reflection in the above-mentioned wavelength region is suppressed by forming the intermediate film, it is not necessary to limit the thickness of the transparent conductive film for preventing the reflection. Thus, the conductivity is not deteriorated. In addition, the average reflectances in a plurality of wavelength regions also can be decreased corresponding to plural kinds of photoelectric conversion layers. Since the photoelectric conversion unit merely has a slight influence on the relative relationship among the respective average reflectances R1, R2, R11, R12, R21, and R22, the values obtained by the measurement in the state where no photoelectric conversion unit is formed on the transparent conductive film may be used as those respective average reflectances.
A substrate for a photoelectric conversion device of the present invention includes a first film with a high refractive index, a second film with a low refractive index, and a transparent conductive film, which are formed on a glass sheet in this order. The first film has a higher refractive index than that of the glass sheet, and the second film has a lower refractive index than those of the first film and the transparent conductive film. The first film has a thickness in the range between 22 nm and 60 nm.